1. Technical Field
The present invention relates to noise reduction techniques used in load drive systems, in particular in the load drive systems using PWM control, for driving a load, such as a motor.
2. Background Art
Among the load drive systems, a motor drive system is used in a wide range of fields. The motor drive system includes a three-phase alternative current motor (may be referred to below simply as a motor), a three-phase inverter that converts a direct current to an alternative current to supply the converted current to the motor, and a control unit that controls the three-phase inverter. For example, as a main motor for use in an electrical vehicle and a hybrid vehicle, an interior permanent magnet motor, in other words a brushless DC motor is employed.
As a method for obtaining a sine wave output voltage from the three-phase inverter, PWM (Pulse Width Modulation) control is generally used. In the PWM control, switching operations are performed at a high speed by switching devices connected in series in each of U-phase, V-phase, and W-phase arms. This contributes heavily to generation of high-frequency switching noise. Furthermore, the motor has parasitic capacitance to ground between the motor itself and a frame ground. Accordingly, when flowing via the parasitic capacitance to ground, the switching noise might cause damage to a bearing of the motor and malfunction of the accessories.
As a way to solve the above problems, Patent Literature 1 discloses a noise reduction method used in a system for driving one motor having two three-phase coil groups by two three-phase inverters, for example. In a structure of Patent Literature 1, a first carrier signal is used in a first three-phase inverter. The first carrier signal is a sawtooth wave (e.g. a waveform formed by repeated cycles of voltage increasing gradually from a first level to reach a second level, and then dropping instantly to the first level). On the other hand, a second carrier signal is used in a second three-phase inverter. The second carrier signal is an inverse sawtooth wave which is inverted with respect to the sawtooth wave. By thus using a pair of the first carrier signal of the sawtooth wave and the second carrier signal of the inverse sawtooth wave, noise is reduced for the following two reasons. A description here focuses on a pair of switching devices constituting identical arms of a same phase in the first and the second three-phase inverters. Note that the “pair of switching devices constituting the identical arms of the same phase” herein refers to, for example, a switching device constituting an upper arm of U-phase in the first three-phase inverter and a switching device constituting an upper arm of U-phase in the second three-phase inverter.
(1) It is assumed that the switching device of the first three-phase inverter transitions from an ON state to an OFF state at a time point when the voltage instantly drops in the sawtooth wave, that is to say, at the time point when the voltage instantly increases in the inverse sawtooth wave. In this case, simultaneously with the transition of the switching device in the first three-phase inverter, the switching device of the second three-phase inverter reversely transitions from the OFF state to the ON state. In this way, the switching devices of the two three-phase inverters simultaneously transition in the opposite state transition directions.
Accordingly, switching noise generated by these switching device operations has opposite directions, whereby noise is cancelled out by each other.
(2) At any time point other than the above time point, using the sawtooth wave and the inverse sawtooth wave prevents the pair of switching devices included in the two three-phase inverters from simultaneously transitioning in the same state transition direction. Accordingly, even when the switching noise is generated due to the operation of each switching device, overlapping between the noise is prevented.
The above phenomena similarly occur in every pair of switching devices corresponding with each other between the first and the second three-phase inverters. As a result, noise is reduced.